Gentlemen,
A rather grusome subject I'm afraid, but how is it that expeditions to wrecks sunk two or three hundred years ago often contain human remains in the shape of skulls and bones but more modern wrecks Hood, Bismarck etc) show no signs of any remains - is it because of the depth that they are at?

I've often wondered about that as well. Why are skeletal remains found in warm climates such as Truk Lagoon but not in the North Atlantic? Yet, just the other day a body of a diver who drowned in Lake Michigan 13 years ago was recently found still in his wet suit. Does the cold water in the Great Lakes preserve organic matter better than the North Atlantic? If so how does that square with Truk? Could it be purely do to the organisms found in the NA but not elsewhere? Quite a puzzle!

Human remains have been found in Vasa, Mary Rose, a number of U-boats sunk in American waters, and the aforementioned remains from ships in Truk lagoon--most of these have been discovered inside the ships during salvage operations or by sports divers exploring wrecks. Bodies which settled on the bottom have largely disappeared due to scavengers and abrasive action of sometimes quite strong bottom currents. I suspect, if one were to explore inside the Titanic or the Bismarck, one would eventually come across remains. The Great Lakes, being very deep and very cold, with sluggish currents compared to the ocean, and being fresh water, appear to much kinder to artifacts and remains than the oceans.

Their shoulders held the sky suspended;
They stood and Earth's foundations stay;
What God abandoned these defended;
And saved the sum of things for pay.

Forensic studies demonstrate that in most marine environments a human corpse will be reduced to skeletal form in less than three weeks, although some degree of articulation may prevail for as long as 18 months. Following the loss of soft tissue, skeletal surfaces are abraded by current-driven sediments. Physical transport of the
remains by currents may result in their impact with hard surfaces to cause additional breakage and dispersal.

Biological activity involving boring, encrusting and scavenging further degrades bones, resulting in advanced deterioration within 12 years, even in cases of the most
durable skeletal parts. As a result, the vast majority of human bones on shipwrecks
are only preserved if and when a victim became trapped below decks, such as beneath cannon and cargo.This prevents dispersion until a sealing layer may be deposited to preserve bone within an anaerobic environment.Therefore, the degree of preservation will depend on how quickly the remains become sediment-inundated and whether such burial seals out oxygen – the main reason that mud and clay are superior to sand for this purpose

Limited skeletal remains typify wrecks located in sandy and hard mud environments, as opposed to fluid mud contexts like the Mary Rose, Kronan and Vasa, where human bones should be expected and are common. The survival of soft tissue only survives in
the most rare of warm or mineral-rich springs and cold,oxygen-depleted environments , such as on the 17th-century La Belle shipwreck in Matagorda Bay, North America.